The biggest problem with alpha blending is that they can not be sorted with Z-buffer. Instead, a separate sorting algorithm has to be applied to alpha polygons and this is very slow, a single alpha blended polygon can take as much time as drawing hundreds of normal polygons. Furthermore, the sorting algorithm is not perfect. Especially when alpha blended polygons "cross" it might make wrong decisions and the polygons would be drawn in incorrect order, bringing something that is supposed to be behind to "pop" in front of something else.